Combustion of coalwater slurry

COMBUSTION AND FLAME 45: 121-135 (1982)

121

Combustion of Coal/Water Slurry E. T. MCHALE, R. S. SCHEFFEE, and N. P. ROSSMEISSL Atlantic Research Corporation, 5390 Cherokee Avenue, Alexandria, Virginia 22314

Dispersions of pulverizedcoal in water, referred to as slurries, have been developedwhichare stable and fluid. They can be pumpedas liquids and burned withoutdewatering in ordinaryequipmentdesignedfor oil. The initial results of a study of the combustionperformanceof coal/water slurries are reported. The fuel will burn as efficiently as pulverizedcoal, and combustionrates approachingthose of heavy oil may be ultimatelyachievable. INTRODUCTION

It is possible to mix high concentrations of finely pulverized coal with water and a small amount of additive to produce a stable slurry fuel. The fuel can be burned similarly to heavy oil using combustion equipment o f ordinary design. We wish to report the first results obtained in a study o f the combustion behavior of this new liquid-like fuel. Research on the concept has been ongoing for some time and involves development and combustion o f slurries of coal in oil and in water. The overall study consists of two phases: Phase I, in which highly loaded coal/oil, coal/oil/water, and coal/ water slurries are being formulated and tested for stability, rheological properties, etc.; and Phase II, in which good candidate slurries chosen from Phase I are burned in an experimental furnace and their combustion performance evaluated. Work on the first phase was reported at two symposia on coaloil mixtures [1, 2]. This paper presents data on the combustion performance of the coal/water slurry only. The slurries formulated for the study are nominally 65/35 (weight) coal to water, stabilized with modified cornstarch. Firings are conducted in a one MMBTUH experimental furnace using a specially designed swirl burner/atomizer which was developed for use with the coal/water slurry. In Copyright O 1982 by The Combustion Institute Published by Elsevier North Holland, Inc. 52 Vanderbilt Avenue, New York, NY 10017

early tests, a small amount of gas assist was usually used, which was omitted in later tests. No change in combustion performance is found when gas is eliminated. Combustion performance is mainly judged by measuring the amount of the combustible matter of the coal that burns and reporting this as combustion efficiency. Efficiencies approaching 90% for the results reported here are unique to the small equipment employed and are about equal to what would be expected for pulverized coal. Another figure of merit would be volumetric heat release rate which falls in the range of 35,000 Btu/fta/hr for the coal/water slurry, comparable to that for pulverized coal combustion. Thermochemical calculations for coal/water slurries are presented. The presence of water represents a relatively small energy penalty. A slurry made from a good coal will have a gross heating value in the range of 10,000 Btu/lb. The heat required to vaporize the water of a 65/35 mixture is about 350 Btu/lb slurry, or about 3.5%. We are aware of other efforts in the United States and in Sweden to develop coal/water slurries, but no reports seem to be available. In Germany and Russia, slurries containing 50-60% powdered coal in water have been burned in large furnaces designed for pulverized coal firing. There are a few reports of the German work and a much larger number of Russian publications. Readers are referred to Refs. [3-5].

0010-2180]82/020121+15502.75

122 DESCRIPTION OF EQUIPMENT Furnace A schematic of the experimental furnace and associated equipment that is being used in the program is shown in Fig. 1, A 9-ft length of schedule 10 pipe, 19.5 in. inside diameter, serves as the combustion chamber. This is positioned in a tank of 3' × 3' cross section through which cooling water continually flows. The water is recycled through a cooling tower at a rate of roughly 500 gph. Ports are located at the end along the top and sides of the furnace for viewing and temperature measurement. A 10-in. i.d. stack is attached near the downstream end. Secondary air is supplied by a North American Model 32316-17-2-3 Turbo Blower which has a capacity approaching 340 SCFM, although rates this high are not used. An inlet duct, 1' diameter × 15' length, is attached to the blower to settle the flow, and rates through the duct are measured with a hot wire anemometer. These flow measurements serve to set approximately the air/fuel ratio during a test but are not accurate enough to be used for exact mass balances. Prior to entering the burner, the air passes through a Chromalox heater of 50 kW capacity. This is adequate to preheat secondary air to 600°F at the highest flow rate used (1000 lb/hr). Other equipment associated with the furnace include a pump and rotometer for No. 2 oil, a metering gear pump for No. 6 oil which is preheated to about 150°F for pumping and >200°F for atomization, a compressor to supply atomizer air and swirl air, rotometers to measure both air flows, and a compressed methane supply for use when assist gas is employed, which is monitored using a critical orifice. Stack gas anlaysis is performed by sampling with an Anderson Sampler (WP-50) at approximately isokinetic rates, and collecting the fly ash (which has a relatively high carbon content) for later proximate analysis. The filtered gas then passes through a condenser to remove water vapor and next through a Beckman nondispersive IR CO 2 Analyzer and Teledyne Model 320 Oxygen Analyzer. Carbon dioxide, carbon monoxide and also oxygen are measured with on Orsat apparatus.

E.T. MCHALE ET AL. The interior of the combustion chamber is lined with refractory, usually five liners of 16" i.d. by 12" length each. These were specially fabricated by Babcock and Wilcox of high-temperaturerated (3000°F) Kaowool.

Burners The scale of the testing of this study, 1 × 106 Btu hr, is at the low end for industrial oil burners. Such small units have narrow passages and orifices, and none was found that could be used with the coal/water slurry which requires a minimum opening size of about 1/8 or 3/16 in. Accordingly, we designed and built our own atomizer/burner which is shown in Fig. 2. In practice, coal/water slurry is expected to be burned in considerably larger units where many types of commercially available burners will be satisfactory. For many of the firings involving No. 2 and No. 6 oil which were burned for baseline data, a North American Model 6526-43-6 Small Heavy Oil Burner was used. The atomizer section of the unit of Fig. 2 is an external, two-fluid atomizer requiring relatively low pressure. It appeared that to get good combustion of coal/water slurry, it would be desirable to add enthalpy to the ignition region. The best arrangement would be one that provided heat without mass addition. In a furnace, this can be accomplished by preheating the secondary air and arranging for good radiation conservation. In addition, recirculation of hot product gases would be helpful even though this represents mass addition. Swirl was favored to achieve the recirculation for the heat addition, which would also provide the benefit of flame stabilization. A swirl burner was therefore combined with the external atomizer. Neither internal air atomization nor pressure atomization was found to work with the coal/ water slurry, at least on the small scale of the present study. The burner/atomizer unit operates as follows, referring to Fig. 2: Slurry from a Moyno pump flows through a 3/16" central tube. Atomizing air enters as shown and flows through a narrow gap, impinging pheripheraUy on the slurry, "shearing" it, and then spraying it out into the swirl generator section of the burner proper.

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Primary swirl air enters tangentially into the burner throat as shown in the lower diagram of the figure. The air issues out the diverging section ( 6 " o.d.) shown on the top diagram and creates a vortex with recirculation back through the central region. It was designed to be versatile as far as experimentation is concerned with the following parameters being variable with relatively little

difficulty: atomizing air gap and pressure; primary swirl air velocity and angle of entry in two dimensions; nozzle size and angle of divergence; position of the atomizer unit within the burner throat.

COMBUSTION OF COAL/WATER SLURRY Swirl number for the burner is estimated to be in the range of 2-3, based on Ref. [6]. Secondary air enters around the outside of the diverging nozzle of Fig. 2 through an annular diffuser of 1/4" gap (not shown). Two styles of diffusers are employed-one in which the secondary air issues into the furnace with only axial flow velocity, and another in which swirl is imparted. The swirl is created by bringing the secondary air into the diffuser tangentially. As will be seen later, the swirl of the secondary air greatly improves slurry combustion. Operating conditions typically are: atomizing air pressure 20 psig with flow of 25 lb/hr; primary swirl air flow of 30 lb/hr; secondary air flow of 500 lb/hr. The methaneassist gas flow rate when used is in the range of 4 lb/hr and the gas is mixed with primary swirl air prior to entering the burner. The value of swirl in pulverized coal combustion has been demonstrated by Beer and Chigier [7] in work performed at the Ijmuiden Laboratories of the International Flame Research Foundation. They used a burner in which swirl could be imparted to primary and secondary air. The primary air swirl was probably not comparable to that of the present work, although the results obtained with their secondary air swirler are comparable to ours. They found that as secondary air swirl strength increased, the point of maximum gas temperature shifted dramatically toward the burner, as did the point of minimum unburned carbon along the axis of the furnace. A pronounced improvement in flame stability was reported.

125 minus 200 mesh and the coarse was 100% minus 50 mesh. Figure 3 shows typical particle size distribution curves for the coarse and fine grinds and for the 55/45 blend. The mass mean diameter of the blended mixture was 53 /a. The specific gravity of the coal/water slurries was approximately 1.25 and viscosity was 800 cp at a shear rate of 3000 sec- 1 . It is notable that the particle blend used in the coal/water slurry produced a distribution of 55% minus 200 mesh (74/a). This represents particle sizes that are greater than usually used in pulverized coal combustion. In the course of the study a number of thermochemical computation s have been performed for slurry and oil fuels and certain of these that are related to coal/water slurries are presented in Figs. 4 and 5.

TABLE 1 Dickerson Proximate-Dry Basis Ash 14.67% Volatile 24.24% Fixed carbon 61.09% Sulfur 1.01% Gross heat value 12,919 Btu/lb Ultimate-Dry Basis Carbon Hydrogen Nitrogen Chlorine Oxygen (diff.)

74.01% 4.46% 1.32%

0.13% 4.40%

PROPERTIES OF COAL The coals used to prepare the slurries were a medium-volatile bituminous of relatively high ash content (referred to as Dickerson) and a highvolatile of low ash (Kentucky). Analyses are based on standard ASTM methods; see Table 1. The exact composition of the coal/water slurry made from Dickerson coal was 65% dry coal, 34% water, and 1% hydroxypropylated cornstarch, which served as stabilizer. The slurry of Kentucky coal was 62% dry coal, 37% water, and 1% starch. In order to obtain the high coal loading in the slurries, a bimodal particle size blend was used which consisted of a 55•45 mixture of coarse-tofine grind. The fine grind was approximately 90%

Kentucky Proximate-Dry Basis Ash 6.19% Volatile 35.00% Fixed carbon 58.81% Sulfur 0.55% Gross heat value 14,109 Btu/lb Ultimate-Dry Basis Carbon Hydrogen Nitrogen Chlorine Oxygen (diff.)

79.25% 5.20% 1.69% 0.20% 7.04%

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Three different coals that were used for various slurries were considered (identified on the plots as 1, 2, 3), encompassing three ash contents. The analyses are listed in Table 2 together with the analysis of the No. 6 oil which was included for comparison. (The Kentucky coal was not included in these computations.) The ash was assumed in the calculations to consist of an equal molar mixture of A12Oa and SiO2 with a composite heat of formation of - 8 3 7 Kcal/mole. Adiabatic flame temperatures calculated from the Atlantic Research thermochemical code are shown in Fig. 4. JANNAF thermodynamic data

were used in all cases. In Fig. 4a, temperatures are plotted versus excess air for the three coals at two different values of secondary air preheat temperature. Typically the temperatures are approximately 200°F lower than those for No. 6 oil. In Fig. 4b, it can be seen that a 10% increase in coal loading of a slurry from 65 to 75% by weight produces approximately 100°F increase in theoretical flame temperature. It may be pointed out that the water that is incorporated into the slurry represents a relatively small energy penalty to the fuel. This is apparent if one considers typical enthalpy values involved. A good bituminous coal will have a gross calorific

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value in the range of 14,000 Btu/lb on a dry basis. In a slurry of 70% loading, the coal will bring of the order of 10,000 Btu to a pound of the slurry. The heat of vaporization of water is close to 1000 Btu/lb, so the 30% water will require about 300 Btu for vaporization, which heat is not recovered. The energy penalty, therefore, of the water is of the order of 3% in a coal]water slurry fuel. A significant consideration if coal/water slurry is to be substituted for heavy oil would be the heat content of combustion gases on a volumetric basis. The size and spacings of boiler tubes are based to a

considerable extent on this factor. Any substantial change in the volumetric enthalpy content of product gas would affect heat transfer in steam generators. In Fig. 5 is plotted the available heat in the combustion gases of 70% slurries of the three coals. It can be seen that these compare favorably with No. 6 oil. Qualitatively the reason for the correspondence between the slurries and oil can be found in the fact that while the heat of combustion of oil on a weight basis is roughly double that of slurry, the air/fuel ratio is also about double. The available heat in terms of Btu/ft 3, for

COMBUSTION OF COAL/WATER SLURRY

129 TABLE 2

Proximate Analysisof Coals-As Received 1

2

"Dickerson. Moisture Volatile matter Ash Fixed carbon Sulfur Gross heat value (Btu/lb)

.

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2.9% 35.7% 7.7% 53.7% 1.10% 13,460

Elkhom" 3.1% 35.0% 3.0% 58.9% 0.60% 14,290

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C H N S O Ash

74.01% 4.46% 1.32% 1.01% 4.40% 14.67%

2

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77.70% 5.06% 1.50% 1.09% 6.71% 7.93% No. 6 Oil Analysis

C H

N S O Gross heat value

example, is then about comparable for the two fuels since oil with the higher calorific value has to heat a greater amount of product (mostly nitrogen). RESULTS The slurries were found to burn satisfactorily, and the many variables associated with the burner, furnace, etc., were investigated and optimized in preliminary tests. The burning performance was characterized by measuring combustion efficiency and examining this dependent variable as a function of air/fuel ratio. There are two ways in which combustion efficiency can be obtained. One is by knowing the total throughput of all air and fuels and then measuring the carbon dioxide or oxygen content of the stack gas. This method could not be employed in the present study because the flow rates of air and fuel could not be measured accurately enough.

79.13% 5.66% 1.50% 0.61% 10.00% 3.10% 84.64% 11.84% 0.02% 0.17% 2.78% 18,402 Btu/lb

The second method by which combustion efficiency can be determined and which was employed in this study is by analyzing the unburned solid residue and using the coal ash as a tracer. (As a simple illustrative example, if a coal with 10% ash were to burn with 80% efficiency, the ash content of the residue would be 35.7%.) In this case, the combustion efficiency (CE) refers to the weight of combustible matter (CM) that is consumed. A general formula can be written relating weight %CE and ash:

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% ash in coal 1 -- (% ash in coal/lO0)

X ( 100 --1). \% ash in residue

(1)

130

E.T. MCHALE ET AL.

There are two sources of error in this method. First, it is not the ash content that should be considered but rather the mineral matter content, the latter typically running somewhat greater than the coal ash. However, this carries over as a negligible error for present purposes. The second source of error is also rather small and results from the fact that the empirical formula of the combustible matter changes during burning. Since the hydrogen content during burning proportionately decreases relative to carbon, and since hydrogen has a very high heat of combustion on a weight basis, any combustion efficiency value cited on a weight HVcoal HVresidu e =

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basis will be less than that on an energy-released basis. However, by far the greatest source of error was found to be not the problems with mineral matter or empirical formula change, but rather that of obtaining a representative sample of unburned residue. We found that the combustible matter content of the residual ash varied widely between fly ash and bottom ash and also with the location of deposit of the bottom ash in the furnace. The two combustion efficiencies (weight and energy) can be related through the heating value (HV) of the unburned residue, given by

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Once combustion efficiency is determined from ash proximate analysis, the amount of excess air that was present during a firing can be determined from stack gas analysis for Oz and/or COz. This procedure is illustrated using the Dickerson coal of the present study which has the empirical formula CHo.71aNo.olsSo.o05100.0446. From the chemical equation (CE)CHo. 718No.o I $So.oo s 100.0446 + (ER)1.16202 + (ER)4.400N 2 (CE)CO 2 + (CE)0.359H20 + (CE)O.O051SO 2 + [(ER)4.400 + (CE)0.0075]N 2 + (ER - CE)1.16202,

(4)

CE(0.162%O 2 - 116.2) 5.562%02 - 116.2

(5)

When methane assist is used it must be taken into account in Eq. (4). The results of tests with the 65/35 slurry of Dickerson coal are presented in Table 3 where most of the entries are self-explanatory. These runs were all made using the secondary air diffuser which injected air with only an axial velocity component. The assist gas is given as weight percent of the slurry. The percent excess air is given relative to total slurry input. The calculated theoretical flame temperatures are based on the actual amount of fuel that burned using energy-released combustion efficiencies. Combustion efficiencies based on energy release are shown as a function of excess air in Fig. 6 as solid points for the data of Table 3. At about 20% excess air, approximately 75% of the energy of the combustible matter of the coal is released. No correlation was found between combustion efficiency and the amount of assist gas.

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131 The results of additional tests with slurries of Dickerson coal plus results of slurries made with the Kentucky coal are presented in Table 4 and shown as open points in Fig. 6. There are several differences between the data of Tables 3 and 4. First, the tests of Table 4 were performed with the secondary air swirl diffuser. This accounts for most of the increase in combustion efficiency that was achieved (apparent in Fig. 6). The second difference is that combustion efficiencies for the tests of Table 4 were obtained by weight averaging the ash content of both bottom and fly ash, whereas for the data of Table 3 only bottom ash was analyzed. Since fly ash usually had a higher ash content than bottom ash, the higher efficiencies of the open point data are partly due to this change in procedure. As best as we are able to estimate, 80% of the increase in combustion efficiency, represented by the dashed line of Fig. 6 over the solid line, was caused by addition of the secondary air diffuser, with about 20% being due to the inclusion of fly ash analysis along with bottom ash. A third important difference is that all the data of Table 4 were obtained with no gas assist. It appears from the open point data of Fig. 6 that slurry of the high-volatile Kentucky coal produced somewhat greater combustion efficiencies than slurry of the medium-volatile Dickerson, by a few percentage points. The duration of the tests was usually held to between 15 and 30 rain in order to conserve slurry. In test No. 5 with the slurry of Kentucky coal, the run time was 80 min with no indication of problems. During these periods the furnace temperature level was approximately constant as measured on the refractory wall with an optical pyrometer. While these measurements (Table 3) are considerably below theoretical flame values, nevertheless they provide an indication of how the combustion is progressing during any given test because they are related to flame temperature and follow changes in it. No signficance can be attached to absolute values of the measured temperatures, as could be readily seen if one were to plot calculated and measured data. The general wall temperature level varies from test to test because the surface of the refractory has an ash or slag coating of varying degree and properties.

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The coal/water slurry burned nicely, and the combustion efficiencies attained are very reasonable for the particle size of the coal and for the small size of the furnace. A better figure of merit at present than combustion efficiency might be volumetric heat release rate, which for a typical firing rate of 700,000 Btu hr and 85% efficiency in an approximately 17-ft a chamber is 35,000 Btu/ft3/hr. It is desirable to make a comparison between the present slurry results and those of dry pulverized coal burning. We have not performed pulverized coal firings in our furnace, so the best that can be done is comparison with data of other studies. One relevant study is that of Beer [8], who burned pulverized anthracite coal and made extensive measurements on burning performance and combustion parameters as a function of particle size and burner type. Another relevant, although

less recent, study is that of Sherman [9]. For present purposes, the Sherman work seems to lend itself better to comparison with the slurry study, especially to our data without the swirl diffuser, because the coals used were bituminous covering a range of volatile and ash content, the main dependent variable measured was essentially combustion efficiency, and the scale of the furnace was much closer to ours, although still larger. In the Sherman study, a 180-ft a refractory wall furnace was used of 15' X 3 ½' X 3 ½'. Firing rates were in the range of 2.7 MMBTUH. Based on data in his paper, a space velocity can be computed of about 0.3 sec- 1 , or a residence time of the order of 3 sec. A comparably computed residence time for the furnace of the present study under typical conditions would be about 1.5 sec. Volumetric heat release rates in the Sherman study were about 20,000 Btu/fta/hr. He burned four coals of

COMBUSTION OF COAL/WATER SLURRY

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133 various ash and volatile matter contents, and reported percentage unburned carbon as a function of particle size and excess air. When one examines the Sherman data and attempts to predict what combustion efficiency would have been obtained in his furnace in the residence time of 1.5 sec and with the coals of the present study, one arrives at a percentage in the seventies or eighties. Or, conversely, the Sherman results indicate that coal burned in pulverized form in our furnace should yield efficiencies about as found for slurry. These very rough comparisons indicate that the coal/ water slurry has burned at least as well as pulverized coal would have been expected to. MISCELLANEOUS CONSIDERATIONS

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Some points of relevance to the present study are worthy of note. The first concerns data that were obtained in the course of the work that bear on the generally accepted mechanism of coal combustion. Coal is usually described as burning in three stages-first, the volatile matter is rapidly released and burns as a gas; second, the remaining char burns via surface reaction, relatively slowly; and third, the CO released in the char reaction burns in the gas phase to CO2, also fairly rapidly. The second stage of the process is considered to be rate limiting, and can be a mass transfer controlled process (as it seems to be for particles >100/am), or a chemical controlled process (as it apparently becomes as particle size falls below 100 /am). In the former case, efficient turbulent mixing promotes reaction, and temperature sensitivity and internal porosity dependence are low. Burn time varies as the square of particle diameter. In the latter case, the reaction is exponentiaUy dependent on temperature, and if the internal surface area becomes great enough, the reaction can become independent of particle size. A thorough review of the subject is given by Mulcahy and Smith [10]. In the present study we have measured low CO/CO2 ratios in the stack gases for the burning of the coal/water slurry, CO almost always being less than 0.1% and CO2 being in the range of 10%. Thus the third stage of burning occurs as generally described. On the other hand, it appears that an

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Fig. 7. Plot of volatile matter content of unburned residue (bottom ash) from slurry fh'ings. Solid points are coal/water and open points are coal/oil/water mixtures. All data are weight percent on dry, ash-free basis. The coal was the Dickerson Medium Volatile Bituminous originally containing 28.4 percent Volatile Matter (DAF). (Two data points for C]W slurry were omitted: 5.6% VM at 67% and 23.1% VM at 83% coal burned.)

assumption of rapid volatile matter release as the first stage does not hold very well. We have collected the b o t t o m ash from the furnace after tests and have found from proximate analyses that it still has a high content of volatile matter. This can be seen in Fig. 7, where the weight percent volatile matter in the b o t t o m ash is shown as a function of the amount of combustible matter burned in the coal. One would expect that when, say, 75% of the total combustible matter of a coal is consumed, that the remaining unburned residue would have very low volatile content. From Fig. 7,

it can be seen that in such an example, the volatile matter of the residue is in the range of about 13% (the scatter exhibited by the data is noted here). The coal was the Dickerson mediumvolatile bituminous, 24.24% VM on a dry basis and 28.40% on a dry, ash-free basis. Another way to view these results is as follows: The original coal contained 28.4% VM and 71.6% FC on a dry, ash-free basis. The unburned residue after 75% of the combustible matter was consumed contained approximately 13% VM. On a unit weight basis, therefore, the VM decreased from

COMBUSTION OF COAL/WATER SLURRY 0.284 to 0.0325 (13% of 0.25), which is close to a 90% loss, while the FC, which was 87% of the residue, decreased from 0.716 to 0.2175 (87% of 0.25), or roughly 70%. The rate of release of volatile matter therefore is faster than the rate of consumption of fixed carbon, but not greatly so. The results from the Kentucky high-volatile coal exhibited a similar pattern (not plotted). The percentage volatile matter on a dry, ash-free basis of the original coal is 37.3%. The unburned bottom residues of the five tests had volatile matter contents, on the same basis, of between 11.4 and 26.4% for coal burnout between 78 and 89%. The early time-temperature history of burning coal in the coal/water slurry is different from what it would be in particle experiments or in pulverized coal combustion. However, it appears that this is not responsible for the behavior that is evident in Fig. 7 because results of tests of a coal] oil/water mixture, burned in separate experiments, fall into the same pattern. In the COW case, the early time-temperature history is completely different than that of C/W slurry, probably resembling that for pulverized coal. Samples of bottom ash were examined by a scanning electron microscope and were found to consist mostly of rounded particles of about 100300/a in diameter, which have considerable macroporosity and appear to be practically hollow inside, as if they had swollen. Pore openings were approximately 2-20 /a. There was no evidence of agglomeration, although this cannot be definitely ruled out. The model of a coal particle burning with progressively decreasing diameter does not hold in the present case. Based on comments made above about mass transfer controlling burning time of particles greater than about 100 ta, it would appear that increased turbulence in pulverized coal burning would produce higher combustion rates. CONCLUSIONS Stable, fluid dispersions containing 62 and 65% pulverized bituminous coal in water have been burned in a nominal 1 X 106-Btu/hr furnace in a manner similar to oil. A burner/atomizer was designed for the small-scale firings, which performed

135 very well. The benefit of swirl was shown by tests in which significant improvement in carbon burnout was realized by imparting a high degree of swirl to the secondary air. The coal/water slurries burned at about the same rate that dry pulverized coal would have been expected to. An interesting finding of the study was that even after large amounts of coal burnout have occurred, the remaining unburned material has a relatively high percentage of volatile matter. This does not appear to be related to the presence of water in the fuel. Another feature of the material that had not completely burned and was recovered as bottom ash is that the particle size was quite large and possessed considerable macroporosity. This research was supported in part by the Pittsburgh Energy Technology Center o f the Department o f Energy under Contract ACO1-77ET13041. Many technical personnel at Atlantic Research contributed valuably to the success o f the combustion phase o f the study, o f whom A. M. McKissick, C. B. Henderson, J. R. Casey, and E. N. Williams are specially acknowledged.

REFERENCES

1. Scheffee,R. S.,Proceedingsof the First lnternational Symposium on Coal-Oil Mixture Combustion, Mitre Corporation, McLean, VA 22102, Dec. 1978. 2. Scheffee,R. S., and McHale,E. T., Proceedingsof the Second International Symposium on Coal-Oil Mixture Combustion, Mitre Corporation, McLean, VA 22102. 3. Das, B. P., Harinathan, K., and Naxasimhan, K. S., J. Mines, Metals and Fuels 22:35 (1974). 4. Schwarz, O., and Merten, E., Gluckauf" 103:225 0967). 5. Delyagin, G. N., Kantorovich, B. V., Kaxachent, V. I.,and Onischenkoa, A, G., UGOL, 39:86 (1964), 6. Beer, J. M., and Chigier, N. A., Combustion Aerodynamics, Applied Science Publishers Ltd., London, 1972. 7. Beer, J. M., and Chigier, N. A., J. Inst. Fuel42:443 (1969). 8. Beer, J. M.,J. Inst.Fuel37:386 (1964). 9. Sherman, R. A.,A.S.M.E. Trans. 56:401 (1934). 10. Mulcahy, M. F. R., and Smith, l.W.,Rev. Pure Appl. Chem. 19:81 (1969).

Received 14 November 1980; revised 20 April 1981